Japanese encephalitis virus is the causative agent of an acute inflammatory illness of the central nervous system found throughout eastern Asia. A formalin-inactivated, mouse-brain derived Japanese encephalitis virus vaccine has been manufactured. As part of our regulatory function we have developed a protocol for in house testing of vaccines for lot release. The final container vaccine lots are tested at four dilution levels: 1:5, 1:25, 1:125, 1:625. Six four-week old female mice are immunized per dilution of vaccine. The reference vaccine is prepared in the same manner and six mice are immunized per dilution of reference. Seven days after the first immunization the mice are immunized again. Six mice serve as unimmunized mice and six serve as sentinel mice to exclude possible interference by mouse-pathogenic agents. The sera of these six mice are submitted for antibody testing and pathology at the conclusion of the study. On the 15th day of the study the mice are bled for serum. The titer of each mouse is determined in a plaque reduction neutralizing antibody test to be performed by CDC. We develop a dose response curve for the vaccine and the reference. The vaccine lot passes if the potency of the vaccine is equal or greater than that of the reference.